In the art it is desirable to produce elastomeric compounds exhibiting reduced hysteresis when properly compounded with other ingredients such as reinforcing agents, followed by vulcanization. Such elastomers, when compounded, fabricated and vulcanized into components for constructing articles such as tires, power belts, and the like, will manifest properties of increased rebound, decreased rolling resistance and less heat-build up when subjected to mechanical stress during normal use.
The hysteresis of an elastomeric compound refers to the difference between the energy applied to deform an article made from the elastomeric compound and the energy released as the elastomeric compound returns to its initial, undeformed state. In pneumatic tires, lowered hysteresis properties are associated with reduced rolling resistance and heat build-up during operation of the tire. These properties, in turn, result in such desirable characteristics as lowered fuel consumption of vehicles using such tires.
In such contexts, the property of lowered hysteresis of compounded, vulcanizable elastomer compositions is particularly significant. Examples of such compounded elastomer systems are known to the art and are comprised of at least one elastomer (that is, a natural or synthetic polymer exhibiting elastomeric properties, such as a rubber), a reinforcing filler agent (such as finely divided carbon black, thermal black, or mineral fillers such as clay and the like) and a vulcanizing system such as sulfur-containing vulcanizing (that is, curing) system.
Previous attempts at preparing reduced hysteresis products have focused upon increased interaction between the elastomer and the compounding materials such as carbon black, including high temperature mixing of the filler-rubber mixtures in the presence of selectively-reactive promoters to promote compounding material reinforcement, surface oxidation of the compounding materials, chemical modifications to the terminal end of polymers using 4,4'-bis(dimethylamino)benzophenone (Michler's ketone), tin coupling agents and the like and, surface grafting.
Various organolithium polymerization initiators are also known in the art. U.S. Pat. No. 3,439,049, owned by the Assignee of record, discloses an organolithium initiator prepared from a halophenol in a hydrocarbon medium.
U.S. Pat. No. 4,015,061 is directed toward amino-functional initiators which polymerize diene monomers to form mono- or di-primary aryl amine-terminated diene polymers upon acid hydrolysis. U.S. Pat. No. 4,914,147 discloses terminal modifying agents including dialkylamino-substituted aromatic vinyl compounds such as N,N'-dimethylamino benzophenone and p-dimethylamino styrene, in rubber compositions having reduced hysteresis characteristics. In U.S. Pat. No. 4,894,409, an amino group-containing monomer, such as 2-N,N-dimethylaminostyrene is polymerized to form an amino group-containing diene based polymer.
Other patents owned by the assignee of record, directed to amine-containing polymerization initiators and terminators include U.S. Pat. Nos. 5,238,893; 5,274,106; 5,329,005; 5,332,810; 5,393,721; 5,496,940; 5,508,333; 5,519,086; 5,521,309; 5,523,371; and 5,552,473. Lawson, et al., in Anionic Polymerization of Dienes Using Homogeneous Lithium Amide (N--Li) Initiators, ACS Preprint, Polymer Division, 37 (2) 1996, at page 728, disclosed that, for carbon black-filled compounds, cyclic amino lithium initiators of a certain size provided elastomeric compounds exhibiting reduced hysteresis.
Precipitated silica has been increasingly used as a reinforcing particulate filler in rubber components of tires and mechanical goods. The reason silica is used in tires is that silica allows one to improve the performance balance between wet traction and rolling resistance, snow/ice traction and, mechanical properties such as wear performance. Silica-loaded rubber stocks, however, have heretofore exhibited relatively poor resilience and high compound viscosity when used without any silane-coupling agent which is somewhat expensive to be widely used in the industry. Even with a certain amount of silane-coupling agent, total balance of the foregoing performances was often not sufficient.
The present invention provides initiators for anionic polymerization which become incorporated into the elastomer chain, as well as various terminators for the polymerization, and produce functional groups which greatly improve the dispersability of silica filler throughout the elastomeric composition during compounding. As will be described hereinbelow, these initiators contain amine groups.
This invention utilizes a combination of silica and polymer with amine producing initiators to increase dispersion of silica filler in diene polymer and copolymer elastomeric compounds, and to thereby reduce the hysteresis of the cured compounds or improve mechanical properties through the interaction (reinforcement) between silica and amine functionalities of the polymer. By this one could even reduce the amount of expensive silane-coupling agent.